Mobile monitor rides the rails to track defects

The RailSurf sled carries eddy-current sensors in the middle of each of its bars that ride the tracks. An inclinometer sits on the horizontal bar perpendicular to the rails, as does a wire sensor for measuring changes in the distance between the rails. A GPS receiver is built into the operator panel. A linear encoder in one of the surf bars records signals from a magnetic wheel that serves as a distance sensor while the device moves down a track.

The RailSurf processor uses five sensors to monitor and record variations in track surface, track height, gauge, and pitch. The resulting information can go onto a removable memory or be read on the operator interface.

An example of the RailSurf in action: Every time the magnetic encoder emits a pulse, an analog/digital converter samples the output of the eddy-current sensors. The sensor signal gets converted to a digital equivalent and then goes to a finite-impulse response filter where it is converted to its frequency-domain equivalent. The processor compares the result against memorized patterns to decide whether it has found a problem.

Schmid Engineering AG in Switzerland designed its RailSurf sled to monitor track rails as an operator pulls it along with a small rail vehicle. It carries several sensors that note such problems as holes on the rails, wavy irregularities in the rail surface called corrugations, and variations in rail gauge and inclination that can make passing trains shudder, shake, and vibrate.

Schmid used a Blackfin processor from Analog Devices Inc. to run the six asynchronous tasks that manage and filter the sensor signals. Engineers created the necessary programs through use of a LabView Embedded Module from National Instruments Corp., Austin, Tex.

The RailSurf sled carries two eddy-current sensors for measuring variations in rail height, a wire sensor for noting changes in the separation between the two rails, a precision inclinometer, and a GPS transceiver. A linear-magnetic encoder used with a magnetic ring serves as an odometer while the sled is pulled along the tracks. Signals from the GPS and odometer pinpoint problems as the system detects them.

In typical operation, the system reads its sensors every time the odometer generates a pulse. The signals feed to an FIR (finite-impulse response) filter prior to storage in removable memory. Filtering operations sift through sensor data to find symptoms of interest. Corrugations, for example, get detected through a Fast-Fourier-transform analysis, which watches for their characteristic 20 to 100-mm wavelength. The system looks for holes on the tracks by comparing eddy-current signals to memorized reference patterns. Designers can tweak filter parameters for track conditions to minimize false alarms.